The present invention relates to a noise reduction treatment for an aircraft cabin, and more particularly to a soundproofing blanket mounting system which reduces aircraft interior noise levels while facilitating maintenance.
Noise develops in an aircraft cabin from several sources. The most common sources are internally or externally mounted moving components, such as a transmission, engine or rotor system. Another source of cabin noise is airflow over various aircraft fuselage components. These components generate vibrations in the aircraft that propagate through the airframe and radiate into the cabin.
Noise may be a particular problem in rotary wing aircraft cabins since the rotor and transmission systems produce a significant amount of vibration directly into the airframe structure. This problem may be more pronounces in rotary wing aircraft than in fixed wing aircraft inasmuch as the dynamic components on a rotary wing aircraft are mounted directly above the cabin.
The main noise problem in helicopter cabins is mid to high frequency gear whine noise from the main transmission. This results in cabin noise vibrations typically from about 350 Hz through 4,000 Hz. In contrast noise vibrations from the main and tail rotor sources are in the 20 Hz to 125 Hz range and are attenuated by up to 40+ dB by the response of the human ear.
Aircraft cabin interiors are generally designed to maintain aircraft interior noise below a certain level predetermined by competitive pressures in the marketplace. For example, executive transport rotary wing aircraft typically provide a design average noise level limit with the environmental control system (fans, vent air and cooling/heating system) turned off of approximately 75 dB SIL4. The SIL4 (Speech Interference Level 4) noise measurement metric is the arithmetic average of the sound pressure levels in the 500, 1000, 2000 and 4000 Hz octave bands. It rates steady noise according to interference with conversation between two people.
Various conventional acoustic absorption systems have been provided to reduce noise levels within the cabin to below desired SIL4 values. One current system is the installation of soundproofing blankets. The blankets are removably attached directly to the airframe frame members in aircraft cabin sections where much of the noise would otherwise be radiated into the cabin interior. More specifically, the soundproofing blankets are attached with hook and loop fastener strips to the frame members in an overlapped arrangement. That is, a first blanket is attached to a frame member and an adjacent blanket is attached to the first blanket. Although relatively effective, there remains acoustic leakage across the attachment intersection of the soundproofing blankets. Vibration grounding also occurs at the blanket intersections due to insufficient clearance between the airframe and interior trim panels. These grounding points and the acoustic leakage combine to result in undesirable cabin noise levels.
Also, because the blankets are overlapped, significant portions of the soundproofing blanket typically including adjacent portions thereof must be removed to provide access to the airframe and avionics systems behind the soundproofing blankets. For example, to remove the left side interior blanket BL, as illustrated in FIG. 6, both interior trim panels I must first be removed; the skeleton frame member S is removed; the right side interior blanket BR is then pulled back in order to access the left side blanket BL.
Accordingly, it is desirable to provide a lightweight, acoustic absorption soundproofing blanket attachment system which minimizes acoustic leakage across intersections yet provides for efficient blanket removal for maintenance access.